Copter: when leaving AUTO only stop mission if running

This resolves an issue in which the mission would not automatically
start from the beginning if it had previously been run to completion
Copter: reset mission when disarming
Copter: minor comment update
AP_NavEKF: Improved divergence detection and reset

Divergence is now detected by looking for very large changes in the gyro
bias. This will cause the filter to be reset and declared unhealthy for
10 seconds.

Don't reset filterDiverged status immediately during reset

Set filterDiverged true if covariance blows up

Add fault status reporting
DataFlash: Add EKF fault status logging
Replay: Add EKF fault status logging message to EKF4